1. Field of the Invention
The present invention relates to the field of well logging. More specifically, the present invention relates to methods for detecting hydrocarbons in reservoirs and fines invasion into formations using nuclear magnetic resonance measurements.
2. Background Art
Oil and gas exploration and production are very expensive operations. Any knowledge about the formations that can help reduce the unnecessary waste of resources in well drilling will be invaluable. Therefore, the oil and gas industry have developed various tools capable of determining and predicting earth formation properties. Among different types of tools, nuclear magnetic resonance (NMR) instruments have proven to be invaluable. NMR instruments can be used to determine formation properties, such as the fractional volume of pore space and the fractional volume of mobile fluid filling the pore space. General background of NMR well logging is described in U.S. Patent No. 6,140,817.
Nuclear magnetic resonance is a phenomenon occurring in a selected group of nuclei having magnetic nuclear moments, i.e., non-zero spin quantum numbers. 1H (proton) is the species commonly detected in NMR well logging because of its natural abundance and sensitivity to NMR measurements. When these nuclei are placed in a magnetic field (Bo, xe2x80x9cZeeman fieldxe2x80x9d), they each precess around the axis of the B0 field with a specific frequency, the Larmor frequency (xcfx89o), which is a characteristic property of each nuclear species (gyromagnetic ratio, Y ) and depends on the magnetic field strength (B0) effective at the location of the nucleus, i.e., xcfx890=Y B0.
Both water and hydrocarbons in the formations produce NMR signals that are detected in well logging. It is desirable that the signals from water and hydrocarbons be separable so that one can identify which regions in the formations have hydrocarbons. However, it is not always easy to distinguish which signals are from water and which are from hydrocarbons. Various methods have been proposed to separately identify water and hydrocarbon signals.
Current NMR methods of hydrocarbon detection may be divided in two categories. The simplest methods involve making a small number of measurements (typically 2) in which just one of the measurement parameters is changed. The difference between the measurements is then interpreted on the basis of known or assumed models for the NMR response of different fluids. The most common techniques of this type are Differential Spectrum (polarization time, WT, is changed) and Shifted Spectrum/Enhanced Diffusion (echo spacing, TE, is changed). These methods take advantages of the fact that water and hydrocarbons often have different relaxation times (T1 and/or T2) and diffusion constants. These methods are outlined in U.S. Pat. No. 6,229,308 B1 issued to Freedman (xe2x80x9cthe Freedman patentxe2x80x9d) and references cited therein. The Freedman patent is assigned to the assignee of the present invention and is hereby incorporated by reference.
The second category of NMR hydrocarbon detection methods is more general and applies forward modeling to suites of NMR data acquired with different parameters, typically TE and WT, although in principle the gradient, G, may also be included as a parameter. There are currently two methods of this type: MACNMR (Slijkerman et al. 1999, SPE 56768) and MRF as disclosed in the Freedman patent.
These NMR hydrocarbon detection methods compare measurements that are made in the same or similar volumes of investigation. When this is not the case, it is nonetheless assumed that all data acquired in the measurement suite may be described by a single set of fluid saturations. None of these methods has exploited variations in NMR response from different depths of investigation.
During well drilling, a fluid (drilling fluid or mud) is pumped into the well. The drilling fluid serves to remove the cuttings from the well and to cool the cutting surfaces of drill bits. The drilling fluids can be water-based muds or oil-based muds. These drilling fluids are typically pumped at high pressure in order to prevent formation fluids from gushing into the well before the well is completed. Because the drilling fluids are at higher pressures than the formation pressures, these fluids may filter into the formation mud filtrate invasion. In addition, fines suspended in the drilling muds may also invade the formation. As used herein, fines refers to very small particles either in muds or mud additives. The extent to which drilling fluids or fines invade the formation depends on several factors: the formation permeability, the pressure differential between the borehole fluids and the formation, the mud type, and the time elapsed since the hole was drilled. Due to the variations in these parameters, the invasion fronts may occur at distances ranging from a few millimeters to several feet into the formation.
The invasion of drilling fluids into the formations is a nuisance for many well logging operations. One either has to sample the formations far away from the wellbore, hoping that the mud filtrate does not reach the region of investigation, or to have a way of differentiating the signals of the mud filtrate from those of the formation fluids. Similarly, invasion of fines from the drilling muds into the formation results in heterogeneity in the vicinity of the borehole. These fine particles can physically plug or bridge across flowpaths in the porous formation, leading to formation damage.
While data processing methods such as the MRF method are capable of separating the mud filtrate components from other components in the NMR measurements, these are post acquisition methods. It is desirable to have NMR logging methods that can take advantage of the mud invasion, in stead of at odds with the mud invasion, so that the presence of hydrocarbons in the formation can be easily identified. In addition, it is desirable to have methods for detecting fines invasion.
One aspect of the invention relates to methods for detecting the presence of hydrocarbons in earth formations. A method for detecting hydrocarbon-bearing zones in a formation penetrated by a wellbore includes acquiring at least two nuclear magnetic resonance measurements, each of the at least two nuclear magnetic resonance measurements acquired from a volume of investigation at a different radial depth from the wellbore; and determining whether the formation bears hydrocarbons by comparing the at least two nuclear magnetic resonance measurements.
Another aspect of the invention relates to methods of well logging. A method for nuclear magnetic resonance logging of a formation penetrated by a wellbore includes providing a nuclear magnetic resonance instrument moveable in the wellbore; acquiring at least two nuclear magnetic resonance measurements, each of the at least two nuclear magnetic resonance measurements acquired at a volume of investigation at a different radial depth from the wellbore; and determining whether the formation bears hydrocarbons by comparing the at least two nuclear magnetic resonance measurements.
Another aspect of the invention relates to methods for detecting fines invasion. A method for detecting fines invasion in a formation surrounding a wellbore includes acquiring at least two nuclear magnetic resonance measurements, each of the at least two nuclear magnetic resonance measurements acquired from a volume of investigation at a different radial depth from the wellbore; and determining whether the fines invasion has occurred by comparing the at least two nuclear magnetic resonance measurements.